Telescopic imaging systems are widely used for imaging scenes through the Earth's atmosphere—ground-based telescopes for astronomical observation and space-based telescopes for aerial surveillance of regions of the Earth's surface. The goal of a typical telescopic imaging system, such as an astronomical telescope, is to achieve an image resolution at the limit permissible by scintillation while imaging the largest solid angle permissible by atmospheric seeing considerations.
A conventional telescopic imaging system includes a long-focal-length objective lens or mirror and a short-focal-length relay lens (i.e., eyepiece). Over time, successively larger aperture telescopes have been developed to improve telescope performance.
Unfortunately, turbulent mixing (caused by effects such as different temperature layers, wind speeds, etc.) can perturb the optical refractive index of the Earth's atmosphere. As a result, as light waves travel through the atmosphere, they can become distorted, leading to image distortions (e.g., aberrations, speckle, etc.) that manifest as optical effects like the blurring and twinkling of stars when viewed from the Earths' surface. They can also lead to impaired image resolution for space-based imaging systems used for aerial surveillance.
Atmospheric perturbation is quantified by the diameter of the “seeing disc,” which is a measure of how severely atmospheric perturbation affects imaging capability. The seeing disc corresponds to the diameter of a blurred image that results from observation of a point-source object through the atmosphere. Theoretically, the seeing limit through the Earth's atmosphere is of order 1 arcsecond (˜0.4 arcseconds has been achieved at high-altitude observatories on small islands such as Mauna Kea or La Palma) without the inclusion of expensive adaptive optics approaches to actively correct for aberrations.
The diameter of the objective lens determines the aperture of the imaging system, which, in turn, determines the brightness and sharpness with which a telescope can image a scene. Image detail (i.e., resolution) and the amount of light captured scale with objective-lens aperture. In other words, telescopic imaging systems having larger apertures enable more image detail and fainter objects to be observed. Unfortunately, a larger aperture lens also results in a larger difference in the optical path of light that travels through the lens on the optical axis from the optical path of light that travels through the lens off the optical axis. Larger aperture lenses, therefore, induce greater aberrations on the light passing them. As a result, complex system designs that include adaptive optics, speckle masking, additional optical surfaces, and/or other atmospheric distortion compensation, are required to achieve diffraction-limited performance making larger such systems more expensive to fabricate.
It is known that restricting the field of view of a large-aperture telescope can provide lower-aberration performance, however. Such an approach has been taken with several such systems that are in operation around the world, such as the 2.5-meter (m) telescope located at Apache Point Observatory, New Mexico. This telescope, used in the Sloan Digital Sky Survey, is equipped with a 120-megapixel camera and, although considered a “wide-field” telescope, has an instantaneous field of view that is limited to approximately 1.5 square degrees of sky. As a result, while this telescope can capture multi-color images of over one-quarter of the sky, these images are obtained over an extremely long period of time due, in part, to the fact that its instantaneous field of view is approximately equal to about eight times the diameter of the full moon. In similar fashion, the “wide-field,” high-resolution ARGUS-IS telescope, includes a restricted instantaneous field of view of approximately 5 arcseconds and a total 45° field of view.
Unfortunately, such prior-art telescopic imaging systems cannot provide high-resolution images over a large instantaneous field of view. This leads to large regions of a scene being unobserved at any given time. As a result, transient events, such as passage of satellites or space debris, super novas, etc., often remain unobserved.
A cost-efficient, high-resolution telescopic imaging system that has a wide instantaneous-field of view, therefore, remains unrealized in the prior art.